MockModuleBuilder: Share root cache between threads

The different build threads all are using the same basic build root
contents, so there's no reason to use separate caches - point the
root cache plugin for mock to a single location. (There's locking
inside Mock for updating the root cache.)

module_build_service/builder/MockModuleBuilder.py

@@ -6,6 +6,7 @@ import os
 import pipes
 import re
 import subprocess
+from textwrap import dedent
 import threading
 
 import dnf
@@ -483,6 +484,11 @@ class MockModuleBuilder(GenericBuilder):
             config = config.replace("$enabled_modules", str(self.enabled_modules))
             config = config.replace("$releasever", str(self.releasever))
 
+            config += dedent("""\
+                config_opts.setdefault('plugin_conf', {})
+                config_opts['plugin_conf'].setdefault('root_cache_opts', {})
+                config_opts['plugin_conf']['root_cache_opts']['dir'] = "{{cache_topdir}}/%s/root_cache/"
+            """) % self.tag_name
 
             mock_cfg_path = os.path.join(self.configdir, "mock.cfg")
             if update_main or not os.path.exists(mock_cfg_path):